The present invention relates to an amplifier and an amplifying method for simultaneously amplifying a plurality of signals having different bands. An amplifier is able to simultaneously amplify a plurality of signals having different bands. This amplifier having a linearizer is described in "Extremely low-distortion multi-carrier amplifier for mobile communication systems", IEICE (the Institute of Electronics, Information and Communication Engineers) Technical Report, RCS90-4, May 24, 1990, for example.
FIG. 1 of the accompanying drawings is a block diagram used to explain an example of such a conventional amplifier, i.e. amplifier having a linearizer. That is, FIG. 1 is a block diagram showing a feed-forward system (hereinafter referred to simply as an F/F system) amplifier for use in a digital modulation system, i.e. linear modulation system such as QAM (Quadrature Amplitude Modulation). In FIG. 1, reference numeral 1 denotes an input terminal, 24 a pilot signal generating section, 25 an adder, 2 a first directional coupler (first distributor or branching filter), 3 a first vector controller, 4 an amplifying section (PA), 5 a second directional coupler (second distributor or branching filter), 9 a first delay element, 10 a subtracter, 12 an attenuator, 27 a third directional coupler (third distributor or branching filter), 15 a second vector controller, 18 an error amplifier, 6 a second delay element, 7 a fourth directional coupler (fourth synthesizer or coupler), 28 a main signal controller, 29 an error signal controller, 26 a fifth directional coupler (first synthesizer or coupler), 30 a bandpass filter (BPF), and 19 an output terminal.
As shown in FIG. 1, a modulation signal is inputted from an input terminal 1. The modulation signal inputted from the input terminal 1 is inputted to the adder 25 through the first directional coupler 2. The first directional coupler 2 branches the inputted signal into two routes. One branched signal is inputted through the first vector controller 3 to the amplifying section 4. The other branched signal is inputted through the first delay element 9 to a subtracted input terminal of the subtracter 10. The signal outputted from the amplifying section 4 is supplied to the second directional coupler 5 and is branched by the second directional coupler 5 into two routes. One branched signal is supplied through the second delay element 6 to the fourth directional coupler 7. The other branched signal is inputted through the attenuator (ATT) 12 to a subtracting input terminal of the subtracter 10. The attenuator 12 matches the level of the inputted signal component with the signal level of the other signal inputted through the first delay element 9 to the subtracter 10 by attenuating the level of the inputted signal component. The first delay element 9 delays the phase of the inputted signal component by the same delay amount as that of the other signal inputted to the subtracter 10 through the attenuator 12.
Although the signal inputted to the amplifying section 4 is amplified to a predetermined output signal level necessary for transmission, the above-mentioned output signal contains an undesired distortion component generated due to a non-linearity of the amplifying section 4 except the necessary main signal. The subtracter 10 calculates a difference between the two inputted signals and extracts this distortion component. At that time, the first vector adjustment section 3 matches the output level (amplitude) and phase of the first delay element 9 to the signal level and phase of the main signal inputted to the subtracting input terminal of the subtracter 10 by adjusting the amplitude and phase of the inputted signal.
This distortion component outputted from the subtracter 10 is inputted through the second vector adjustment section 15, the error amplifier 18 and the third directional coupler 27 to the fourth directional coupler 7.
This distortion component becomes a signal which has a phase opposite to that of the signal inputted to the fourth directional coupler 7 through the second delay element 6. The fourth directional coupler 7 synthesizes the two inputted signals to cancel the distortion components out with respect to each other, thereby resulting in the linearization being effected. The output signal from the fourth directional coupler 7 is supplied through a fifth directional coupler 26 to a bandpass filter (BPF) 30, and an undesired component is eliminated from this output signal by the bandpass filter 30 and a resultant signal is outputted from the output terminal 19. A signal line from the input terminal 1, the first directional coupler 2, the first vector adjustment section 3, the amplifying section 4, the second directional coupler 5, the second delay element 6, the fourth directional coupler 7 to the output terminal 19 in FIG. 1 will hereinafter be referred to as a main signal transmission section. Also, other circuit portions than the main signal transmission section will be referred to as a distortion compensation section.
An example of an adjustment method for eliminating a distortion component in FIG. 1 will be described next.
One method will be described below. When a communication device such as a transmitter is activated, two non-modulated waves having close frequencies are inputted from the input terminal 1 as test signals, and the output from the subtracter 10 and the signal from the output terminal 19 are monitored. Then, according to this method, the vector adjustment section 3 and the vector adjustment section 15 are adjusted while these signals are being monitored. However, according to this method, the vector adjustment section 3 and the vector adjustment section 15 cannot be adjusted during the communication device is operated (during communication) so that this method cannot follow the characteristic change caused by a temperature fluctuation caused after data was transmitted in actual practice.
To solve the problem of the first method, there is proposed a second method in which a pilot signal is inserted near the transmission band at the input terminal side. This second method will hereinafter be described with reference to FIGS. 1 and 2.
FIG. 2 is a diagram showing a signal spectrum obtained after a pilot signal was inserted. In FIG. 2, a horizontal axis f represents a frequency, a vertical axis represents an electric power level, O.L. represents an output signal level, pit represents a pilot signal and D represents a distortion component.
Referring to FIG. 1, the pilot signal plt from the pilot signal generating section 24 is supplied to the adder 25, in which it is added to the signal from the first directional coupler 2. In FIG. 1, the third directional coupler 27 supplies the output signal of the amplifier 18 to the fourth directional coupler 7, and also branches a part of the above output signal and supplies the signal thus branched to the main signal controller (CONT1) 28. While monitoring the output signal from the subtracter 10, the main signal controller 28 adjusts the first vector adjustment section 3 in such a manner that the power of the monitored signal becomes minimum. Further, the fifth directional coupler 26 supplies the output signal of the third directional coupler 27 to the bandpass filter 30, and also branches a part of the output signal and supplies the signal thus branched to the error signal controller 29. While monitoring the output signal of the fifth directional coupler 26, the error signal controller 29 adjusts the second vector adjustment section 15 in such a manner that the pilot signal power plt of the monitored signal becomes minimum. In this manner, it is possible to eliminate the distortion component during the communication.
When the signals of the up-link (from the remote station to the master station) and the down-link (from the master station to the remote station) are simultaneously inputted and amplified like a link amplifier for use in a relation station for transmitting signals from the base station (master station) to the remote station or vice versa by using a plurality of frequency bands as described above, it is necessary to linearize over a very wide frequency band covering all of a plurality of different frequency bands.